Surface-emission semiconductor laser device

ABSTRACT

A surface-emitting semiconductor laser device includes a semi-insulating substrate, a layer structure with a bottom multilayer reflector, an n-type cladding layer, an active layer structure for emitting laser, a p-type cladding layer and a top multilayer reflector with a dielectric material, consecutively formed on the semi-insulating substrate, the active layer structure, the p-type cladding layer and the top multilayer reflector, configuring a mesa post formed on a portion of the n-type cladding layer, the p-type cladding layer or the p-type multilayer reflector. The surface-emitting semiconductor laser includes a p-side electrode formed on another portion of the p-type cladding layer, and an n-side electrode formed on another portion of the n-type cladding layer. The n-side electrode includes a substantially uniform Au film and AuGeNi film or AuGe film consecutively formed on the n-type cladding layer, and an alloy is formed between said Au film and said AuGeNi film or AuGe film.

The present application is a continuation-in-part of application Ser.No. 11/392,493, filed Mar. 30, 2006, which is a continuation-in-part ofapplication Ser. No. 11/109,667, filed Apr. 20, 2005, which is acontinuation of application Ser. No. 10/246,553, filed Sep. 19, 2002,which is a continuation of application Ser. No. 09/546,691, filed Apr.10, 2000. The entire contents of the above applications are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating asurface-emission semiconductor laser device and, more particularly, to amethod for fabricating a surface-emission semiconductor laser devicewhich has an improved characteristic of injected current/applied voltageand thus is suited for use in an optical data transmission or opticalcommunication.

2. Description of the Related Art

Surface-emission semiconductor laser devices, particularly thoseimplemented on GaAs substrates, attract a large attention as lightsources for use in optical communication systems in the field of datacommunication. The surface-emission semiconductor laser devicefabricated on a GaAs substrate generally includes a pair ofmulti-layered semiconductor mirrors, or distributed Bragg reflectors(DBRs), each formed of a plurality of pairs of AlGaAs layers each havinga mixed crystal structure, and a GaAs active layer structure sandwichedbetween the pair of DBRs for emission of laser perpendicular to the mainsurface of the GaAs substrate.

The surface-emission semiconductor laser device, which emits laserperpendicular to the main surface of a substrate, has an advantage ofintegration feasibility wherein a large number of laser elements arearranged two-dimensionally on the single substrate. Accordingly, thesurface-emission semiconductor laser devices are particularly suitablefor applications to parallel optical information processing, such asoptical interconnection and optical computing, or a large-scale paralleloptical transmission by taking advantage of the parallel optical beams.

The surface-emission semiconductor laser device implemented on an n-typeGaAs substrate includes a p-type DBR, which has a larger resistivitycompared to the n-type DBR due to the inherent characteristic thereofand a smaller occupied area compared to the n-type DBR due to the devicestructure. This raises a problem of higher electric resistance of thep-type DBR, which causes a smaller injected current with respect to avoltage applied between the electrodes.

FIG. 6 shows the surface-emission semiconductor laser device describedin the above-mentioned publication. The semiconductor laser device 50includes a p-type GaAs substrate 52, a p-type DBR 54 composed ofGaAs/AlAs layers formed on the entire surface of the p-type substrate52, a column structure formed thereon and including an active layerstructure 56, an n-type DBR 58 composed of GaAs/AlAs layers and ann-side electrode 60, and a p-side electrode 62 formed on the p-type DBR54 separately from the column structure. The laser device 50 emits laserperpendicularly to and through the bottom surface of the substrate 52,as shown in FIG. 6.

In a laser device formed on a p-type substrate, in general, an n-typecompound semiconductor layer constitutes the top of the epitaxiallayers. In the example of FIG. 6, the top epitaxial layer is the n-typeGaAs layer or the AlAs layer implementing the n-type DBR 58.

Since the surface-emission semiconductor laser device generally includesepitaxial layers having complicated structures formed on a small area ofthe main surface of the substrate, the contact area between the n-typecompound semiconductor layer and the n-side electrode is inherentlysmall. This highlights the importance in reduction of the contactresistance between the n-type compound semiconductor layer and then-side electrode in the surface-emission semiconductor laser device. Ina conventional surface-emission semiconductor laser having the p-typesubstrate, however, the reduction of the contact resistance is notsatisfactory, and thus the operational voltage of the surface-emissionsemiconductor laser device is higher than the desired voltage.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide a method for fabricating a surface-emission semiconductor laserdevice on a p-type substrate, the surface-emission semiconductor laserdevice having a smaller contact resistance between the n-type compoundsemiconductor layer and the n-side electrode.

The present invention provides a method for fabricating asurface-emission semiconductor laser device, including the steps offorming consecutively a p-type distributed Bragg reflector (DBR), anactive layer structure for emitting laser, and an n-type DBR whichoverlie a main surface of a p-type semiconductor substrate, forming ann-side electrode overlying the n-type DBR, and forming a p-sideelectrode on a bottom surface of the p-type semiconductor substrate, then-side electrode forming step including the steps of forming an Au film,forming an AuGeNi film or AuGe film on the Au film, and forming an alloybetween the Au film and the AuGeNi film or AuGe film.

In accordance with the method of the present invention, the presence ofthe Au film improves the adherence between the compound semiconductorlayer and the n-side electrode, which in turn improves an injectioncurrent vs. applied voltage characteristic.

The above and other objects, features and advantages of the presentinvention will be more apparent from the following description,referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a surface-emitting semiconductor laser device according toan embodiment of the present invention.

FIG. 2 shows a surface-emitting semiconductor laser device according toan embodiment of the present invention.

FIG. 3 shows a surface-emitting semiconductor laser device according toan embodiment of the present invention.

FIG. 4 shows a surface-emitting semiconductor laser device according toan embodiment of the present invention.

FIG. 5 shows a surface-emitting semiconductor laser device according toan embodiment of the present invention.

FIG. 6 is a sectional view of a conventional surface-emissionsemiconductor laser device.

FIGS. 7A to 7D are sectional views of a surface-emission semiconductorlaser device in consecutive steps of fabrication thereof in a methodaccording to an embodiment of the present invention.

FIG. 8 is a graph for showing an injected current vs. applied voltagecharacteristic in the semiconductor laser device of the embodiment.

FIG. 9 is a sectional view of the structure of a surface-emissionsemiconductor laser device according to an example of the presentinvention.

FIG. 10 is a sectional view of a surface-emission semiconductor laserdevice according to an example of the present invention.

FIG. 11 is a sectional view of a surface-emission semiconductor laserdevice according to an example of the present invention.

DESCRIPTION OF THE EMBODIMENTS

In a typical surface-emission semiconductor laser device of the priorart having a p-type substrate, the n-side electrode is generally made ofAuGe or AuGeNi formed on the n-type compound semiconductor layersepitaxially grown on the p-type substrate (refer to, for example, K.Matusda, et al., IEEE Photonics Technol. Lett., vol. 8, pp. 494, 1996).

The inventors noticed the excellent adherence between a compoundsemiconductor layer and an Au film, and had an idea of interposing an Aufilm between the n-type compound semiconductor layers and the n-sideelectrode made of AuGe(Ni) for reduction of the contact resistancetherebetween. The inventors fabricated a number of samples ofsurface-emission semiconductor lasers on p-type GaAs substrates, thesemiconductor lasers having a variety of n-side electrodes made ofmetallic or alloy films. The inventors then conducted experimentsthereto by measuring injected current vs. applied voltagecharacteristics.

Experiments

The samples were fabricated by using the method shown in FIGS. 7A to 7D.In FIG. 7A, 28 combinations of a p-type Al_(0.2)Ga_(0.8)As layer and ap-type Al_(0.9)Ga_(0.1)As layer and an additional p-typeAl_(0.9)Ga_(0.1)As layer were epitaxially grown on a p-type GaAssubstrate 12 by using a MOCVD (metal-organic chemical vapor deposition)technique to form a p-type DBR 14.

Subsequently, an undoped lower Al_(0.3)Ga_(0.7)As cladding layer 16, anactive layer structure 18 implemented as a GaAs/Al_(0.2)Ga_(0.8)Asmultiple quantum well (MQW) structure including three 7-nm-thick GaAsquantum well active layers, and an undoped upper Al_(0.3)Ga_(0.7)Ascladding layer 20 were consecutively grown on the p-type DBR 14.

Thereafter, 25 combinations of an n-type Al_(0.2)Ga_(0.8)As layer and anAl_(0.9)Ga_(0.1)As layer were grown on the upper cladding layer 20 toform an n-type DBR 22, followed by epitaxial growth of an n-type caplayer 24 on the top Al_(0.2)Ga_(0.8)As layer of the n-type DBR 22,thereby obtaining the structure shown in FIG. 7A.

A SiNx film 26 was then deposited on the entire area by using a plasmaenhanced CVD technique, followed by formation of a photoresist film 28on the SiNx film 26 and patterning thereof by using an ordinaryphotolithographic technique to form an etching mask 28. The etching mask28 had an annular opening which encircles a central circular pattern 30having a diameter of 30 μm.

The SiNx film 26 was then subjected to a reactive ion etching (RIE)technique by using CF₄ gas and the etching mask 28 to form an opening 32in the SiNx film 26. The resultant structure is shown in FIG. 7B.

The etching mask 28 was then removed, followed by wet etching of then-type cap layer 24, n-type DBR 22, upper cladding layer 20, activelayer structure 18 and lower cladding layer 16 by using the SiNx film 26as an etching mask and an etchant including a mixture of phosphoricacid, hydrogen peroxide and water. Thus, an annular portion of thep-type DBR 14 was exposed, and a column structure including the n-typecap layer 24, n-type DBR 22, upper cladding layer 20, active layerstructure 18 and lower cladding layer 16 was formed on the central areaof the p-type DBR 14.

After removing the SiNx mask 26 by using a RIE technique, another SiNxfilm 34 was deposited on the entire area. Then, a portion of the top ofthe another SiNx film 34 covering the column structure was removed byusing a photolithographic technique and a RIE technique to form anemission window 36 having a diameter of 10 μm. Subsequently, an n-sideelectrode 38 was formed by using an electron beam evaporation and apatterning technique to have an emission opening therein correspondingto the emission window 36. The n-side electrode 38 was made of aplurality of metallic films and had an annular area for currentinjection.

After the bottom of the GaAs substrate 12 was polished to obtain athickness of 100 μm for the GaAs substrate 12, a p-side electrode 40made of AuZn film was formed by evaporation onto the polished bottomsurface of the GaAs substrate 12. A surface-emission semiconductor laserdevice 10 was thus achieved after an alloying treatment of the n-sideelectrode 38 and the p-side electrode 40.

A plurality of samples for the surface-emission semiconductor laserdevice 10 of FIG. 7D were fabricated which included different structuresof the n-side electrode. The samples had n-side electrodes including atleast one of three films of metals or alloys, as shown in Table 1. Inthe table, each column shows the material for the corresponding film andthe thickness (nm) thereof in parentheses.

1 TABLE 1 Sample No. First film Second film Third film Sample #1 Au (30)AuGeNi (50) Au (200) Sample #2 Au (30) AuGe (50) Au (200) Sample #3 Au(250) Sample #4 AuGeNi (50) Au (200) Sample #5 Ti (100) Pt (200) Au(200)

The AuGeNi film in Samples #1 and #4 included Ni at a ratio below 1%, Geat about 12%, and the balance of Au at about 88%. The AuGe film inSample #2 included Au at about 88% and the balance of Ge at about 12%.

The adherence between the metallic films and the compound semiconductorlayer in each of Samples #1 to #5 was inspected at the surface of then-side electrode after annealing the samples for three minutes at atemperature of 420.degree. C. in a nitrogen ambient. Samples #1 to #3exhibited an alloy structure of the n-side electrode, Samples #4exhibited unevenness or irregularity at the surface of the n-sideelectrode, and Samples #5 did not exhibit an alloy structure, without asubstantial change of the color or shape of the n-side electrode afterthe annealing.

More specifically, it is judged that Samples #1 and #2, wherein an Aufilm was interposed between the AuGeNi or AuGe film and the compoundsemiconductor layer, had a higher adherence between the electrode andthe semiconductor layer. It is also judged that Sample #2, wherein theelectrode was formed by a single Au film, had a higher adherence. On theother hand, it is judged that Sample #4, wherein an Au film was formedon the AuGeNi film by evaporation, had poor adherence between theelectrode and the compound semiconductor layer and that the annealtreatment caused agglomeration of metals in the electrode. It is alsojudged that Sample #5, wherein a Ti film was interposed, had a pooradherence because substantially no alloy was formed in the electrode.

From the above results for adherence, it is concluded that the Au filmhas an excellent adherence with respect to the compound semiconductorlayer. It is also concluded that the Au film interposed between theAuGeNi or AuGe film and the compound semiconductor layer improves theadherence therebetween. That is, the excellent adherence between the Aufilm and the compound semiconductor layer improves the adherence betweenthe electrode and the compound semiconductor layer even in the case of asmall thickness of the Au film as low as 30 nm or below.

The next experiments were such that a voltage was applied between thep-side electrode and the n-side electrode in Samples #1 to #5 formeasuring injected current vs. applied voltage characteristics. Theresults are shown in FIG. 8, wherein the applied voltage is plotted onabscissa and the injected current between the p-side electrode and then-side electrode is plotted on ordinate.

As understood from FIG. 8 by comparing the injected current amongSamples #1 to #5, Samples #1 and #2 had larger current compared toSamples #3 to #5. This means that the electric resistance of the n-sideelectrode is smaller in Samples #1 and #2 having AuGe(Ni)/Au structurein the n-side electrode than in other samples.

Although Sample #3 had an excellent adherence between the n-sideelectrode and the compound semiconductor layer which is similar to theadherence in Samples #1 and #2, the alloy formed between the compoundsemiconductor and the metals in the electrode in Sample #3 had a higherelectric resistivity compared to the alloy formed by the combination ofAuGe(Ni)/Au in Samples #1 and #2.

Sample #4 had a highest resistance among the samples due to the factthat metals in the electrode in Sample #4 were agglomerated by theannealing treatment and an alloy was not substantially formed betweenthe metals and the compound semiconductor layer.

It is known from the prior art that the metals of the electrode inSample A5 do not form an alloy by an annealing treatment. For achievinga lower resistance electrode in the Ti/Pt/Au structure, it may beconsidered that the compound semiconductor layer is heavily doped withimpurities. However, the improvement in the resistance by the heavydoping is limited compared to the improvement of resistance in thealloy.

From the above results, it is concluded that the Au film interposedbetween the AuGeNi or AuGe film and the compound semiconductor layerbefore an annealing treatment achieves a low-resistance electrode whilesuppressing agglomeration of metals in the electrode during theannealing treatment even in the case of a small thickness for the Aufilm.

Based on the findings as described above, the surface-emissionsemiconductor laser device of the present invention has an Au filminterposed between the AuGeNi or AuGe film and the compoundsemiconductor layer.

Now, the present invention is more specifically described with referenceto the preferred embodiment thereof.

The method of the preferred embodiment of the present invention isbasically same as the method used for fabricating the samples asdescribed above with reference to FIGS. 7A to 7D.

More specifically, in the method of the preferred embodiment of thepresent invention, 28 combinations of a p-type Al_(0.2)Ga_(0.8)As layerand a p-type Al_(0.9)Ga_(0.1)As layer and an additional p-typeAl_(0.9)Ga_(0.1)As layer are epitaxially grown on a p-type GaAssubstrate 12 by using a MOCVD technique to form a p-type DBR 14, asshown in FIG. 7A.

Subsequently, an undoped lower Al_(0.3)Ga_(0.7)As cladding layer 16, anactive layer structure 18 implemented as a GaAs/Al_(0.2)Ga_(0.8)Asmultiple quantum well (MQW) structure including three 7-nm-thick GaAsquantum well active layers, and an undoped upper Al_(0.3)Ga_(0.7)Ascladding layer 20 are consecutively grown on the p-type DBR 14.

Thereafter, 25 combinations of an n-type Al_(0.2)Ga_(0.8)As layer and anAl_(0.9)Ga_(0.1)As layer are grown on the upper cladding layer 20 toform an n-type DBR 22, followed by epitaxial growth of an n-type caplayer 24 on the top Al_(0.2)Ga_(0.8)As layer of the n-type DBR 22,thereby obtaining the structure shown in FIG. 7A.

A SiNx film 26 is then deposited on the entire area by using a plasmaenhanced CVD technique, followed by formation of a photoresist film 28on the SiNx film 26 and patterning thereof by using an ordinaryphotolithographic technique to form an etching mask 28. The etching mask28 has an annular opening which encircles a central circular pattern 30having a diameter of 30 μm.

The SiNx film 26 is then subjected to a reactive ion etching (RIE)technique by using CF₄ gas and the etching mask 28 to form an opening 32in the SiNx film 26. The resultant structure is shown in FIG. 7B.

The etching mask 28 is then removed, followed by wet etching of then-type cap layer 24, n-type DBR 22, upper cladding layer 20, activelayer structure 18 and lower cladding layer 16 by using the SiNx film asan etching mask and an etchant including a mixture of phosphoric acid,hydrogen peroxide and water. Thus, an annular portion of the p-type DBR14 is exposed, and a column structure including the n-type cap layer 24,n-type DBR 22, upper cladding layer 20, active layer structure 18 andlower cladding layer 16 is formed on the central area of the p-type DBR14.

After removing the SiNx film 26 by using a RIE technique, another SiNxfilm 34 is deposited on the entire area. Then, a portion of the top ofthe another SiNx film 34 covering the column structure is removed byusing a photolithographic technique and a RIE technique to form anemission window 36 having a diameter of 10 μm. Subsequently, a40-nm-thick Au film, a 50-nm-thick AuGeNi film and a 200-nm-thick Aufilm are consecutively deposited on the entire surface by electron beamevaporation, followed by patterning thereof to form a n-side electrode38 having an emission window 36 and an annular area for currentinjection.

After the bottom of the GaAs substrate 12 is polished to obtain athickness of 100 μm for the GaAs substrate 12, a p-side electrode 40made of AuZn film is formed on the polished bottom surface of the GaAssubstrate 12 by evaporation. A surface-emission semiconductor laserdevice 10 is achieved after an alloying treatment of the n-sideelectrode 38 and the p-side electrode 40.

In the above embodiment, the n-side electrode 38 and the p-sideelectrode 40 are subjected to the annealing treatment for three minutesat temperature of 420.degree. C. in a nitrogen ambient for alloying themetals in the electrodes. The AuGeNi film in the n-side electrode 39includes a small amount of Ni which is below 1%, Ge at about 12% and thebalance of Au at about 88%.

The n-side electrode 38 may be formed of, for example, a 30-nm-thick Aufilm, a 50-nm-thick AuGe film and a 200-nm-thick Au film instead of thestructure as described above. In this case, the AuGe film preferablyincludes Au at about 88% and Ge at about 12%.

Since the above embodiments are described only for examples, the presentinvention is not limited to the above embodiments and variousmodifications or alterations can be easily made therefrom by thoseskilled in the art without departing from the scope of the presentinvention. For example, an additional layer may be interposed betweentwo of layers or at least one of layers may be omitted or modified inthe laser device of the embodiment.

FIG. 1 shows a surface-emitting semiconductor laser device according toa first embodiment. The laser device 100A includes: an n-type orsemi-insulating substrate 101, and a layer structure including a bottomn-type or undoped multilayer reflector 102, an n-type lower claddinglayer 103, an active layer structure 104 for emitting laser, a p-typeupper cladding layer 106 and a top multilayer reflector 107 including anundoped semiconductor or dielectric material, which are consecutivelyformed on the n-type or semi-insulating substrate 101. A portion of thelayer structure including the active layer structure 104, the p-typecladding layer 106 and the top multilayer reflector 107 configures amesa post 108 formed on a portion of the n-type cladding layer 103. Thep-type cladding layer 106 or the p-type multilayer reflector 107includes therein a current confinement oxide layer 105. The topmultilayer reflector 107 is formed on a portion of the p-type claddinglayer 106. A p-side electrode 110 is formed on another portion of thep-type cladding layer 106 encircling the top multilayer reflector 17. Ann-side electrode 109 is formed on another portion of the n-type claddinglayer 103 encircling the active layer structure 104. The n-sideelectrode 109 includes a substantially uniform Au film and an AuGeNifilm or AuGe film consecutively formed on the n-type cladding layer 103,and an alloy formed between the Au film and the AuGeNi film or AuGefilm.

FIG. 2 shows a surface-emitting semiconductor laser device according toa second embodiment. The laser device 100B includes a p-type substrate101, and a layer structure including a p-type bottom multilayerreflector 102, a p-type lower cladding layer 103, an active layerstructure 104 for emitting laser, an n-type upper cladding layer 106,and a top multilayer reflector 107 including a dielectric material orsemi-insulating material or undoped semiconductor, which areconsecutively formed on the p-type substrate 101.

A portion of the layer structure including the p-type cladding layer103, the active layer structure 104, the n-type cladding layer 106 andthe top multilayer reflector 107 configures a mesa post 108 formed on apotion of the p-type bottom multilayer reflector 102. The p-type lowercladding layer 103 includes therein a current confinement oxide layer105. The top multilayer reflector 107 is formed on a portion of then-type cladding layer 106. A p-side electrode 10 is formed on the bottomsurface of the p-type substrate 101. An n-side electrode 109 is formedon another portion of the n-type cladding layer 106 encircling the topmultilayer reflector 107. The n-side electrode 109 includes asubstantially uniform Au film and an AuGeNi film or AuGe filmconsecutively formed on the n-type cladding layer, and an alloy formedbetween the Au film and the AuGeNi film or AuGe film.

FIG. 3 shows a surface-emitting semiconductor laser device according toa third embodiment. The laser device 100C includes: an n-type substrate101; and a layer structure including an n-type bottom multilayerreflector 102 an n-type lower cladding layer 103, an active layerstructure 104 for emitting laser, a p-type upper cladding layer 106 anda p-type multilayer reflector 120, which are consecutively formed on then-type substrate 101. The p-type cladding layer 106 or the p-typemultilayer reflector 120 includes therein a current confinement oxidelayer 105.

A tunnel junction structure 110 is formed on the p-type multilayerreflector 120, and includes a heavily-doped p-type layer 110 a and anoverlying heavily-doped n-type layer 110 b. An n-type top multilayerreflector 111 is formed on the tunnel junction structure 110. A firstn-side electrode 112 is formed on a portion of the n-type top multilayerreflector 111. A second n-side electrode 113 is formed on the bottomsurface of the n-type substrate 101. The first n-side electrode 112includes a substantially uniform Au film and an AuGeNi film or AuGe filmconsecutively formed on the n-type top multilayer reflector 111, and analloy formed between the Au film and the AuGeNi film or AuGe film.

FIG. 4 shows a surface-emitting semiconductor laser device according toa fourth embodiment. The laser device 100D includes: an n-type substrate101; and a layer structure including an n-type bottom multilayerreflector 102, an n-type lower cladding layer 103, an active layerstructure 104 for emitting laser, a p-type upper cladding layer 106 anda p-type multilayer reflector 120, which are consecutively formed on then-type substrate 101. The p-type cladding layer 106 or the p-typemultilayer reflector 111 includes therein a current confinement oxidelayer 105.

A tunnel junction structure 110 is formed on the p-type multilayerreflector 120 and includes a heavily-doped p-type layer 110 a and anoverlying heavily-doped n-type layer 110 b. An n-type contact layer 114is formed on the tunnel junction structure 110. A top multilayerreflector 111 including a dielectric material or semi-insulatingmaterial or undoped semiconductor is formed on a portion of the n-typecontact layer 114. A first n-side electrode 112 is formed on anotherportion of the n-type contact layer 114 encircling the top multilayerreflector 111. A second n-side electrode 113 is formed on the bottomsurface of the n-type substrate 101. The first n-side electrode 112includes a substantially uniform Au film and an AuGeNi film or AuGe filmconsecutively formed on the n-type cladding layer, and an alloy formedbetween the Au film and the AuGeNi film or AuGe film.

A method for manufacturing a semiconductor device according to a fifthembodiment is directed to forming a surface-emitting laser device shownin FIG. 5. The method of the present embodiment includes the step ofconsecutively forming, on a semi-insulating substrate 101, a bottommultilayer reflector 102, a p-type lower cladding layer 103, an activelayer structure 104 for emitting laser, an n-type upper cladding layer106, and a top multilayer reflector 107 including a semi-insulatingsemiconductor or dielectric material. A portion of the layer structureincluding the top multilayer reflector 107, n-type cladding layer 106and active layer thus formed is then etched to form a mesa structure108.

A p-side electrode 110 is then formed on the p-type cladding layer 103.In an alternative, the p-side electrode 110 may be formed on the bottommultilayer reflector 102 after forming the p-type cladding layer 103 inthe mesa structure 108. An n-side electrode 109 is then formed on then-type cladding layer 106. The step of forming the n-side electrode 109includes forming a substantially uniform Au film on the n-type claddinglayer 106, followed by forming an AuGeNi film or AuGe film on the Aufilm, and lastly forming an alloy formed between the Au film and theAuGeNi film or AuGe film. The n-side electrode 109 may be formed on thetop multilayer reflector 107 instead.

In the above embodiment, the step of forming the alloy in the n-sideelectrode 109 may include annealing the Au film and the AuGeNi film orthe AuGe film.

In the above embodiment, the step of forming the n-side electrode 109may further include forming another Au film on the AuGeNi film or theAuGe film.

FIG. 9 is a sectional view of the structure of a surface-emissionsemiconductor laser device according to a first example of the presentinvention. The laser device, generally designated by numeral 10A,includes a p-GaAs substrate 12, and a layer structure including a bottomDBR 14, a p-type lower cladding layer 16, a QW active layer structure18, an n-type upper cladding layer 20 and a top DBR 22, which areconsecutively layered on the p-GaAs substrate 12. The bottom DBR 14includes 35 p-Al_(0.9)Ga_(0.1)As/n-Al_(0.2)Ga_(0.8)As layer pairs,wherein each layer has a thickness of λ/4n, given λ and n being theemission wavelength and refractive index, respectively. The top DBRincludes 22 i-Al_(0.9)Ga_(0.1)As/i-Al_(0.2)Ga_(0.8)As layer pairs,wherein each layer has a thickness of λ/4n, given λ and n being theemission wavelength and refractive index of each layer. It is to benoted here that the term “i-Al_(0.9)Ga_(0.1)As”, for example, means asemi-insulating semiconductor Al_(0.9)Ga_(0.1)As, which is undoped withimpurities.

The bottom DBR 14 is such that an Al_(0.9)Ga_(0.1)As layer therein nearthe QW active layer structure 18 is replaced by an AlAs layer 42, whichis subjected to selective oxidation by oxidizing the Al component in theAlAs layer 42 in the peripheral region 48A thereof to leave the centralun-oxidized-Al region 48B. The un-oxidized-Al region 48B of the AlAslayer 42 functions as a current injection region, whereas theoxidized-Al region 48A of the AlAs layer 42 functions as a currentconfinement region.

A portion of the bottom DBR 14 including the oxidized AlAs layer 42, aswell as the lower p-type cladding layer 16, QW active layer structure 18and upper n-type cladding layer 20 among the layer structure isconfigured as a lower mesa-post 48 having a diameter of 30 μm, forexample, by using a photolithographic and etching technique.

The top DBR 22 is also configured as an upper mesa-post 48A having adiameter smaller than the diameter of the lower mesa-post 48 asdescribed above. A SiNx layer 42 is formed on top and side of themesa-post 48 and on the portion of the bottom DBR 14 etched duringforming the mesa-post 48. The SiNx layer 42 has an opening. An annularn-side electrode 38 is formed on a top portion of the lower mesa-post 48where the upper mesa-post 48A is not formed, and contacts the n-typecladding layer 20 via the opening of the SiNx film 42. A p-sideelectrode 40 is formed on the bottom surface of the p-GaAs substrate 12after polishing the bottom surface.

The configuration of the first example allows the top DBR 22 to beformed from a material having a higher transmission factor for the laserwithout requesting an electric conductivity.

FIG. 10 is a sectional view of a surface-emission semiconductor laserdevice according to a second example of the present invention. The laserdevice, generally designated by numeral 10B, includes a substrate 18A,and a layer structure including a bottom DBR 14, a p-type lower claddinglayer 16, a QW active layer structure 18, an n-type upper cladding layer20 and a top DBR which are consecutively layered on the substrate 18A.The bottom DBR 14 includes p-Al_(0.9)Ga_(0.1)As/p-Al_(0.2)Ga_(0.8)Aslayer pairs, wherein each layer has a thickness of λ/4n, given λ and nbeing the emission wavelength and refractive index, respectively. Thetop DBR 22 includes 25 n-Al_(0.9)Ga_(0.1)As/n-Al_(0.2)Ga_(0.8)As layerpairs, wherein each layer has a thickness of λ/4n, given λ and n beingthe emission wavelength and refractive index, respectively.

In this example, the n-side electrode 38 and p-side electrode 40A areformed to overlie the same main surface of the substrate 18A, wherebythe substrate 18A need not have an electric conductivity and thus may bean insulating substrate.

The bottom DBR 14 is such that a Al_(0.9)Ga_(0.1)As layer therein nearthe QW active layer structure 18 is replaced by an AlAs layer 42, whichis subjected to selective oxidation by oxidizing the Al component in theAlAs layer 42 in the peripheral region 48A thereof to leave the centralun-oxidized-Al region 48B. The un-oxidized-Al region 48B of the AlAslayer 42 functions as a current injection region, whereas theoxidized-Al region 48A of the AlAs layer 42 functions as a currentconfinement region.

A portion of the bottom DBR 14 including the oxidized AlAs layer 42among the layer structure is configured as a first mesa-post 48B havinga diameter of 70 μm, for example, by using a photolithographic andetching technique. Another portion of the bottom DBR 14 including theoxidized AlAs layer 42, as well as the lower cladding layer 16, QWactive layer structure 18 and upper cladding layer 20 among the layerstructure is configured as a second mesa-post 47C having a diametersmaller than the lower mesa-post 48B. An annular n-side electrode 38 isformed on top of the top DBR 22.

On a portion of the first mesa-post 48B on which the second mesa-post47C is not formed as well as on the etched portion of the bottom DBR 14,a p-side electrode 40A is formed with an intervention of a SiNx layer 42and contacts the bottom DBR 14 via an opening of the SiNx layer 42. Itis to be noted that the bottom DBR 14 may be made of a semi-insulatingmaterial in the region through which the operating current of the laserdevice does not pass. The semi-insulating material may be asemiconductor undoped with impurities.

It is also noted that the first mesa-post 48B may have a uniformdiameter from the oxidized AlAs layer 42 to the lower cladding layer 16,and the p-side electrode 40A is formed on the lower cladding layer 16.

In an alternative of the configuration of the present embodiment, then-side electrode 38, upper cladding layer 20 and the top DBR 22 may bereplaced by p-type semiconductors, and the p-side electrode 40A andbottom cladding layer may be replaced by an n-type semiconductors.

FIG. 11 is a sectional view of a surface-emission semiconductor laserdevice according to a third example of the present invention. The laserdevice, generally designated by numeral 10C, includes a substrate 18B,and a layer structure including a bottom DBR 44, a p-type lower claddinglayer 16, a QW active layer structure 18, an n-type upper cladding layer20 and a top insulating DBR 46 which are consecutively layered on thesubstrate 18B. The bottom DBR 44 includesn-Al_(0.9)Ga_(0.1)As/n-Al_(0.2)Ga_(0.8)As layer pairs, wherein eachlayer has a thickness of λ/4n, given λ and n being the emissionwavelength and refractive index, respectively. The top DBR 46 includes25 Si/SiO layer pairs, wherein each layer has a thickness of λ/4n, givenλ and n being the emission wavelength and refractive index,respectively.

In this example, the n-side electrode 38A and p-side electrode 40A areformed to overlie the same main surface of the substrate 18B, wherebythe substrate 18B need not have an electric conductivity and thus may bean insulating substrate or a conductive substrate.

The bottom DBR 44 is such that an Al_(0.9)Ga_(0.1)As layer therein nearthe QW active layer structure 18 is replaced by an AlAs layer 42, whichis subjected to selective oxidation by oxidizing the Al component in theAlAs layer 42 in the peripheral region 48A thereof to leave the centralun-oxidized-Al region 48B. The un-oxidized-Al region 48B of the AlAslayer 42 functions as a current injection region, whereas theoxidized-Al region 48A of the AlAs layer 42 functions as a currentconfinement region.

A portion of the bottom DBR 44 among the layer structure is configuredas a first mesa-post 47D having a diameter of 70 μm, for example, byusing a photolithographic and etching technique. Another portion of thebottom DBR 44 including the oxidized AlAs layer 42, as well as the lowercladding layer 16, QW active layer structure 18 and upper cladding layer20 among the layer structure is configured as a second mesa-post 48Ehaving a diameter smaller than the lower mesa-post 48B. The upper DBR 46is configured as a third mesa-post having a diameter smaller thediameter of the second mesa-post. A p-side electrode is formed on aportion of the upper cladding layer 20 of the second mesa-post 48E wherethe third mesa-post 48F, or top DBR 46, is not formed.

An annular n-side electrode 38A is formed on a portion of the bottom DBR44 of the first mesa-post 47D where the second mesa-post 48E is notformed. A portion of the bottom DBR 44 which does not configure acurrent path between the n-side electrode and the p-side electrode maybe formed from a semi-insulating semiconductor material undoped withimpurities.

In an alternative, the first mesa-post 47D may include the whole bottomDBR 44 including the oxidized AlAs layer 42, and may be formed to have auniform diameter. In this case, the n-side electrode 38A may be formedon the lower cladding layer 16 to overlie the same.

1. A surface-emitting semiconductor laser device comprising: an n-typeor semi-insulating substrate; a layer structure including a bottommultilayer reflector, an n-type cladding layer, an active layerstructure for emitting laser, a p-type cladding layer and a topmultilayer reflector including a semi-insulating semiconductor ordielectric material, which are consecutively formed on said n-type orsemi-insulating substrate, said active layer structure, said p-typecladding layer and said top multilayer reflector configuring a mesa postformed on a portion of said n-type cladding layer, said p-type claddinglayer or said p-type multilayer reflector including therein a currentconfinement oxide layer, said top multilayer reflector being formed on aportion of said p-type cladding layer; a p-side electrode formed onanother portion of said p-type cladding layer; and an n-side electrodeformed on another portion of said n-type cladding layer, said n-sideelectrode including a substantially uniform Au film and an AuGeNi filmor AuGe film consecutively formed on said n-type cladding layer, and analloy formed between said Au film and said AuGeNi film or AuGe film. 2.A surface-emitting semiconductor laser device comprising: a p-typesubstrate; a layer structure including a p-type bottom multilayerreflector, a p-type cladding layer, an active layer structure foremitting laser, an n-type cladding layer and a top multilayer reflectorincluding a dielectric material or semi-insulating material or undopedsemiconductor, which are consecutively formed on said p-type substrate,said p-type cladding layer, said active layer structure, said n-typecladding layer and said top multilayer reflector configuring a mesa postformed on a potion of said p-type bottom multilayer reflector, saidp-type cladding layer including therein a current confinement oxidelayer, said top multilayer reflector being formed on a portion of saidn-type cladding layer; a p-side electrode formed on a bottom surface ofsaid p-type substrate; and an n-side electrode formed on another portionof said n-type cladding layer, said n-side electrode including asubstantially uniform Au film and an AuGeNi film or AuGe filmconsecutively formed on said n-type cladding layer, and an alloy formedbetween said Au film and said AuGeNi film or AuGe film.
 3. Asurface-emitting semiconductor laser device comprising: an n-typesubstrate; a layer structure including an n-type bottom multilayerreflector, an n-type cladding layer, an active layer structure foremitting laser, a p-type cladding layer and a p-type multilayerreflector which are consecutively formed on said n-type substrate, saidp-type cladding layer or said p-type multilayer reflector includingtherein a current confinement oxide layer; a tunnel junction structureformed on said p-type multilayer reflector and including a heavily-dopedp-type layer and an overlying heavily-doped n-type layer; an n-type topmultilayer reflector formed on said tunnel junction structure; a firstn-side electrode formed on at least a portion of said n-type topmultilayer reflector; and a second n-side electrode formed on a bottomsurface of said n-type substrate, said first n-side electrode includinga substantially uniform Au film and an AuGeNi film or AuGe filmconsecutively formed on said n-type top multilayer reflector, and analloy formed between said Au film and said AuGeNi film or AuGe film. 4.A surface-emitting semiconductor laser device comprising: an n-typesubstrate; a layer structure including an n-type bottom multilayerreflector, an n-type cladding layer, an active layer structure foremitting laser, a p-type cladding layer and a p-type multilayerreflector which are consecutively formed on said n-type substrate, saidp-type cladding layer or said p-type multilayer reflector includingtherein a current confinement oxide layer; a tunnel junction structureformed on said p-type multilayer reflector and including a heavily-dopedp-type layer and an overlying heavily-doped n-type layer; an n-typecontact layer formed on said tunnel junction structure; a top multilayerreflector including a dielectric material or semi-insulating material orundoped semiconductor and formed on a portion of said n-type contactlayer; a first n-side electrode formed on another portion of said n-typecontact layer; and a second n-side electrode formed on a bottom surfaceof said n-type substrate, said first n-side electrode including asubstantially uniform Au film and an AuGeNi film or AuGe filmconsecutively formed on said n-type cladding layer, and an alloy formedbetween said Au film and said AuGeNi film or AuGe film.
 5. A method forfabricating a surface-emitting semiconductor laser device comprising thesteps of: consecutively forming on a semi-insulating substrate a bottommultilayer reflector, a p-type cladding layer, an active layer structurefor emitting laser, an n-type cladding layer, and a top multilayerreflector including a semi-insulating semiconductor or dielectricmaterial; forming a p-side electrode on said p-type cladding layer orsaid bottom multilayer reflector; and forming an n-side electrode onsaid n-type cladding layer, said n-side electrode forming step includingforming a substantially uniform Au film on said n-type cladding layer orsaid top multilayer reflector, followed by forming an AuGeNi film orAuGe film on said Au film, and lastly forming an alloy formed betweensaid Au film and said AuGeNi film or AuGe film.
 6. The method accordingto claim 5, wherein said alloy forming step includes annealing said Aufilm and said AuGeNi film or said AuGe film.
 7. The method according toclaim 5, wherein said n-side electrode forming step further includesforming another Au film on said AuGeNi film or said AuGe film.